Role of saturated and unsaturated zone in soil disposal of septic tank effluent

The guidelines for the use of the septic tank – soil absorption system (ST–SAS) in the Province of British Columbia are very specific in regard to the separation distance between the ground surface and the groundwater table (minimum 1.2 m), and between the tile field and perimeter drains or ditches (minimum 3.0 m). A pilot-scale experiment employing waterproof channels filled with a saturated soil was used to evaluate the scientific basis for these guidelines. Septic tank effluent was applied to unsaturated columns and the inlet end of the channels and samples were taken at different points in the channels. Measurements were made of total and fecal coliforms, chemical oxygen demand, ammonia, nitrate, and orthophosphate. Coliform reductions within the channels were at least 30 000-fold with total coliform numbers generally lying below 200 coliforms/100 mL and fecal coliforms generally less than 50 coliforms/100 mL. Varying degrees of nitrification occurred in the unsaturated columns, resulting in relativel...